In vivo and in vitro noncovalent association of excised α1(i) amino-terminal propeptides with mutant pnα2(I) collagen chains in native mutant collagen in a case of Ehlers-Danlos syndrome, type VII

The cause of the Ehlers-Danlos syndrome Type VII (EDS VII) is considered to be defective removal of the amino-terminal propeptide (N-propeptide) of Type I procollagen due to deficiency of procollagen N-proteinase, the enzyme responsible for the normal proteolytic excision of this precursor-specific domain. Molecules retaining the N-propeptide (pN-collagen molecules) are thought to cause defective fibrillogenesis and cross-linking which eventuate in dramatic joint laxity and joint dislocations, the clinical hallmark of this variety of EDS. Recent studies demonstrate that some EDS VII patients harbor small deletions of either the pro-α1(I) or pro-α2(I) chain of Type I procollagen. We have found an 18-amino acid deletion (due to exon outsplicing) in a mutant pro-α2(I) chain from such a patient. The deleted peptide is the junctional segment (N-telopeptide) linking the α2(I) N-propeptide and major triple helical domains; loss of this short segment results in union of these latter domains and produces a shortened pNα2(I) chain. Directly extracted tissue collagen and pepsin-digested fibroblast collagen contain this mutant pNα2(I) chain and normal α1(I) chains, but not pNα1(I) chains, indicating that the relatively larger α1(I) N-propeptide is excised from the related α1(I) chains. The fate of this α1(I) N-propeptide was unclear and therefore whether or not the intact N-propeptide was, in fact, retained in native mutant collagen was also unclear. In this paper, we describe morphologic, chemical, and immunochemical studies which indicate that the α1(I) N-propeptide is retained in noncovalent association with the mutant pNα2(I) chain in native mutant collagen molecules both in vivo and in vitro. In both instances, the α1(I) N-propeptides are proteolytically cleaved from the related α1(I) chains. These data suggest that retention of a partially cleaved, but essentially intact N-propeptide in mutant collagen may play a role in the pathogenesis of this disease.